Vehicle-based strategy for removing urea deposits from an SCR catalyst

ABSTRACT

A method is provided for controlling regeneration of an SCR catalyst. The method includes coordinating the regeneration duration and temperature (e.g., longer/shorter regenerations and/or lower/higher temperatures) to the urea deposit loading. In this way, improved regeneration may be achieved due to the particular nature of urea deposits on SCR catalysts.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/743,239, filed May 2, 2007, the entire contents of which areincorporated herein by reference for all purposes.

FIELD

The present application relates to a system and method for improving theperformance of a vehicle's exhaust gas aftertreatment system;specifically the application relates to a system and method forregenerating a urea-based selective catalytic reduction (SCR) catalystto improve NOx conversion efficiency.

BACKGROUND

Vehicles powered with diesel engines have to be equipped with leanexhaust aftertreatment devices such as a urea-based Selective CatalyticReduction (SCR) catalyst to reduce NOx emissions. The urea-based SCRcatalyst requires the injection of urea to provide ammonia (NH₃) as areductant for NOx reduction. The ammonia generation from ureadecomposition follows mainly two steps, as shown below:NH₂—CO—NH₂(gas or liquid)=NH₃(gas)+HNCO(gas)  (1)HNCO (gas)+H₂O(gas)=NH₃+CO₂(gas)  (2)

The first step (1) is a thermal decomposition reaction and the secondstep (2) is a hydrolysis reaction. The thermal decomposition of urea isslow and is the rate limiting step at a temperature below 300° C.Therefore, when the exhaust temperature is below 300° C., the spray ofurea solution to the SCR catalyst may be deposited mostly as urearelated compounds on the SCR without being fully decomposed.

Diesel exhaust gas temperature may be low (e.g., less than 300° C.) whena diesel vehicle is driven in urban driving cycles. Thus, urea relateddeposits may be formed on the SCR catalyst. The urea related depositsmay plug pores in the washcoat and reduce the catalyst surface area andthe catalyst's activity. Further, clogging on the catalyst may increasethe back pressure over the catalyst, and thus negatively impact engineperformance and increase fuel consumption.

U.S. Pat. No. 6,892,530 discloses a method to regenerate a urea-basedSCR catalyst by maintaining its temperature above the boiling point ofhydrocarbons to remove hydrocarbon deposits. However, the inventorsherein have recognized that the above approach may not remove urearelated deposits sufficiently. For example, the temperature and timeinterval for removing hydrocarbon deposits used in the '530 patent maynot be effective to remove the urea-based deposits.

SUMMARY

According to one aspect, a method for operating an aftertreatment devicecoupled downstream of an internal combustion engine is provided. Theaftertreatment device includes a urea-based selective catalyst reduction(SCR) catalyst. The method comprises establishing a threshold ureadeposit accumulation for regeneration of the SCR catalyst; determiningwhether the threshold urea deposit accumulation in the SCR catalyst hasbeen met; and in response to the determination, regenerating the SCRcatalyst by maintaining the SCR catalyst at or above a predeterminedregeneration temperature for a predetermined interval. In one example,the interval and/or predetermined temperature may be varied with anamount of urea deposit.

According to another aspect, a method to regenerate a urea-basedselective catalyst reduction (SCR) catalyst coupled downstream of aninternal combustion engine is provided. The method comprises estimatingurea deposit accumulation in the SCR catalyst to determine a timing forregeneration of the SCR catalyst to remove the urea depositaccumulation; raising a temperature of the SCR catalyst to apredetermined temperature and maintaining the predetermined temperaturefor a predetermined time interval to regenerate the SCR catalyst; andadjusting an exhaust flowrate entering the SCR catalyst to improve heattransfer in the SCR catalyst and removal of the urea depositaccumulation.

According to yet another aspect, a system for a vehicle comprises aninternal combustion engine; a urea based selective catalyst reduction(SCR) catalyst coupled downstream of the internal combustion engine; anda controller configured to adjust operating parameters to regenerate theSCR catalyst, where the controller varies a temperature at which the SCRcatalyst is regenerated with an amount of urea deposited on the SCRcatalyst, and where the controller varies a duration for which the SCRcatalyst is regenerated with said amount of urea deposited. For example,by coordinating the regeneration duration and temperature (e.g.,longer/shorter regenerations and/or lower/higher temperatures) to theurea deposit loading, improved regeneration may be achieved due to theparticular nature of urea deposits on SCR catalysts.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic depiction of an exemplary embodiment of anengine.

FIG. 1B is an alternative exemplary embodiment of an engine.

FIG. 2 is a schematic depiction of an aftertreatment device coupleddownstream of an internal combustion engine.

FIG. 3 illustrates an exemplary method 200 for operating anaftertreatment device coupled downstream of an internal combustionengine according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

FIGS. 1A and 1B are schematic diagrams of an engine. As shown in FIG.1A, internal combustion engine 10, comprising a plurality of cylinders,one cylinder of which is shown in FIG. 1A, is controlled by electronicengine controller 12. Engine 10 includes combustion chamber 30 andcylinder walls 32 with piston 36 positioned therein and connected tocrankshaft 40. Combustion chamber 30 is shown communicating with intakemanifold 44 and exhaust manifold 48 via respective intake valve 52 andexhaust valve 54. Intake manifold 44 is also shown having fuel injector80 coupled thereto for delivering liquid fuel in proportion to the pulsewidth of signal FPW from controller 12. Both fuel quantity, controlledby signal FPW and injection timing are adjustable by controller 12,described below. Fuel is delivered to fuel injector 80 by a fuel system(not shown) including a fuel tank, fuel pump, and fuel rail (not shown).

Controller 12 is shown in FIG. 1A as a microcomputer includingmicroprocessor unit 102, input/output ports 104, read-only memory 106,random access memory 108, and a data bus. Controller 12 is shownreceiving various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including engine coolanttemperature (ECT) from temperature sensor 112 coupled to cooling sleeve114, a measurement of manifold pressure (MAP) from pressure sensor 116coupled to intake manifold 44, a measurement (AT) of manifoldtemperature from temperature sensor 117, an engine speed signal (RPM)from engine speed sensor 118 coupled to crankshaft 40. An aftertreatmentdevice 20 is coupled to an exhaust manifold 48 and is described withparticular reference to FIG. 2.

Referring now to FIG. 1B, an alternative embodiment is shown whereengine 10 is a direct injection engine with injector 80 located toinject fuel directly into cylinder 30.

Referring now to FIG. 2, the aftertreatment device 20 comprises aurea-based Selective Catalytic Reduction (SCR) catalyst 14, which iscapable of reducing NOx in an oxygen rich environment. Reductant, suchas aqueous urea, is stored in a storage vessel (not shown) and deliveredthrough a reductant delivery system 16 coupled to exhaust systemupstream of SCR catalyst 14. The reductant is metered out by a pumpthrough a control valve, where both the pump and the valve arecontrolled by controller 12. Alternatively, any other suitable meansknown to those skilled in the art to supply reductant to an exhaust gasaftertreatment device may be used. A heater 22 may be coupled to the SCRcatalyst to provide heat for a regeneration of the SCR catalyst.

Controller 12 is configured to control an operation of the SCR catalyst,such as regeneration, based on information measured by sensors or basedon estimated parameters. For example, the temperature of the SCRcatalyst may be measured by a temperature sensor 24, and may be used asone of the parameters to estimate the urea deposit accumulation or tocontrol the SCR catalyst regeneration, etc. One or more NOx sensors,such as an upstream NOx sensor 17 and a NOx sensor 18 down stream of theSCR catalyst, are coupled in the path of the exhaust gas entering andexiting the SCR catalyst. The outputs of these sensors are read bycontroller 12 and may be used to estimate urea deposit on the SCRcatalyst.

While typically two NOx sensors are provided, it will be appreciatedthat only one NOx sensor may be provided. For example, in oneembodiment, only downstream NOx sensor 18 is provided and the controller12 is configured to estimate the urea deposit on the SCR catalyst basedon the output from downstream NOx sensor 18 along with one or moreengine operation and urea injection parameters. For example, enginespeed, load, exhaust gas temperature or any other parameter known tothose skilled in the art to affect engine NOx production, may be used bycontroller 12 in the estimation of NOx entering the SCR catalyst.Further, a pressure drop across the SCR catalyst may be used bycontroller 12 to estimate the urea deposit. One or more pressure sensors26 and 27 may be used to measure the pressure drop. Furthermore, anexhaust flow rate entering the SCR catalyst may be measured by a flowmeter 29.

Oxidation catalyst 13 is coupled upstream of the SCR catalyst and may bea precious metal catalyst, preferably one containing platinum. Theoxidation catalyst exothermically combusts hydrocarbons (HC) in theincoming exhaust gas from the engine thus supplying heat to rapidly warmthe SCR catalyst 14. The temperature of the SCR catalyst may be raisedby controller 12 through retarding injection timing, increasing EGR andintake throttling, or any other suitable means known to those skilled inthe art to increase the temperature of the exhaust gas. Alternatively,in a direct injection engine as shown in FIG. 1B, extra hydrocarbons maybe delivered to the oxidation catalyst for the SCR catalyst warm-up byin-cylinder injection during either or both of a power or exhaust strokeof the engine. Particulate filter 15 is coupled downstream of the SCRcatalyst 14. As described below, heat from particulate filterregeneration may be used to heat the SCR catalyst.

It should be noted that various embodiments for the aftertreatmentdevice may be available. For example, although depicted downstream, itwill be appreciated that particulate filter 15 may be disposed upstreamof SCR catalyst.

FIG. 3 illustrates an exemplary method 200 for operating anaftertreatment device coupled downstream of an internal combustionengine according to one embodiment of the present disclosure. Asdescribed above, the aftertreatment device may include a urea-based SCRcatalyst, an oxidation catalyst and a particulate filter. Method 200 maybe used to regenerate the SCR catalyst by removing urea deposit or ureadeposit accumulation. First, at 202, the method includes establishing athreshold urea deposit accumulation for regeneration of the SCRcatalyst. As described above, urea deposit accumulation may plug thepores in the washcoat of the SCR catalyst and reduce the effectivecatalyst area and catalyst activities. The threshold urea depositaccumulation may be a level at which the SCR catalyst cannot achieve adesired efficiency to remove NOx.

Next, at 204, the method determines whether a temperature of the SCRcatalyst is less than a predetermined temperature. As described above,the first step of ammonia generation from urea decomposition is athermal decomposition reaction. Low temperature may decrease thedecomposition rate of urea and cause the formation of urea related solidcompounds. As urea solution is sprayed onto the SCR catalyst, ureadeposit accumulation may be formed at a low exhaust temperature. Forexample, when the exhaust temperature is below 300° C., urea depositsmay be formed, which may clog the catalyst. Thus, the method may use atemperature as a threshold to determine a need for SCR regenerationassociated with the urea deposit accumulation. In some embodiments, thepredetermined temperature may be in the range of 300-350° C. Preferably,the predetermined temperature may be 350° C. When the temperature isbelow 350° C., urea deposits may vaporize slowly and tend to accumulateon the SCR catalyst. Alternatively, step 202 may proceed to step 206directly without step 204. The SCR catalyst regeneration may bedetermined by threshold urea deposit accumulation.

Further, it should be noted that when the temperature is below a certainlevel, urea injected into the SCR catalyst may not be decomposed at arate sufficiently high enough to be used as a reductant. Thus, in someembodiments, urea injection may be disabled at or below a low exhausttemperature, such as 165° C.

Next, at 206, the method determines whether the threshold urea depositaccumulation has been met. Determining that the threshold urea depositaccumulation on the SCR catalyst has been met includes estimating ureadeposit accumulation and determining that the estimated urea depositaccumulation meets or exceeds the threshold urea deposit accumulation.Various approaches may be used to estimate urea deposit accumulation.For example, at 208, determining whether the threshold has been met maybe accomplished by estimating the urea deposit accumulation based onoperating conditions of the SCR catalyst. For example, the urea depositaccumulation may be a function of operating variables, such as operatingtemperature, urea flow rate, exhaust flow rate, exhaust pressure,pressure differences across the SCR catalyst, ammonia to NOx ratio (aparameter for controlling amount of urea injected into the exhaustsystem), etc. Thus, the urea deposit accumulation may be estimated by acontroller based on one or more variables described above.

Alternatively, at 210, determining whether the threshold has been metmay be accomplished by estimating the urea deposit accumulation. Becausethe urea deposit accumulation may affect the efficiency of the SCRcatalyst, the urea deposit accumulation may be a function of NOxemissions. Thus, the urea deposit accumulation may be estimatedaccording to an output of NOx sensors. For example, NOx conversionefficiency may be estimated based on an output from NOx sensor 17upstream of the SCR catalyst as shown in FIG. 2 and an output from NOxsensor 18 downstream of the SCR catalyst also shown in FIG. 2.Alternatively, NOx sensor 17 may be eliminated and the amount of NOx inthe exhaust gas mixture entering the SCR catalyst can be inferred basedon engine speed, load, exhaust gas temperature or any other suitableparameter known to those skilled in the art to affect engine NOxproduction. Alternatively, or additionally, one or more of the operatingvariables, such as operating temperature, urea flow rate, exhaust flowrate, exhaust pressure, pressure differences across the SCR catalyst,equivalence ratio, etc. may be used along with the NOx sensor outputs inthe estimation of urea deposit accumulation.

Alternatively, at 212, determining whether the threshold has been metmay be accomplished by detecting a pressure drop across the SCRcatalyst. Because the urea deposit accumulation may increase the backpressure over the catalyst, a pressure drop across the SCR catalyst maycorrespond with the urea deposit accumulation in the SCR catalyst. Thus,determining that the threshold urea deposit accumulation in the SCRcatalyst has been met may include detecting whether the pressure dropacross the SCR catalyst meets or exceeds the predetermined pressure dropthreshold corresponding to the threshold urea deposit accumulation.

Next, at 214, the method determines whether particulate filterregeneration or a desulfation event is imminent. If the answer to step214 is “No”, SCR regeneration is initiated at 216. The method includesraising the temperature of the SCR catalyst to a predeterminedgeneration temperature and maintaining the regeneration temperature fora predetermined time. Urea deposits may vaporize slowly at temperaturesat 350° C. or below. On the other hand, urea deposit may vaporizequickly at temperatures at 400° C. or greater. In some embodiments, theregeneration temperature may be in the range of 360-450° C. At thistemperature range, urea deposits can be significantly removed.

The increase of the SCR catalyst temperature may be accomplished byvarious methods. At 218, raising the temperature may be accomplished byinjecting fuel into the oxidation catalyst to generate an exothermicreaction, which may generate sufficient heat to raise the SCR catalysttemperature. The amount of fuel injected and duration of the injectionmay be determined by a prestored map based on engine operatingconditions, such as engine speed, load, catalyst temperature, exhaustgas temperature, etc., or any other suitable method. Alternatively, at220, the temperature may be raised by heating SCR catalyst using anelectrical heater. At 222, raising the temperature of the SCR catalystmay be accomplished by adjusting engine related measures, such asretardation of injection timing, increasing exhaust gas recirculation(EGR), closing an intake throttle, etc.

The duration of the regeneration may be determined such that the ureadeposit accumulation may be removed to an extent that any remaining ureadeposits would not affect the performance of the SCR catalyst. In someembodiments, the regeneration time may be at least two minutes, which issufficient time for effective SCR catalyst regeneration.

Next, at 224, the method may include adjusting the flow rate of exhaustgas entering the SCR catalyst. In some conditions, exhaust flow rate orspace velocity may be decreased to improve the effectiveness of removingthe urea deposit on the SCR catalyst.

If, on the other hand, at 214 the answer is “yes”, at 226, the methodincludes delaying the SCR catalyst regeneration until the completion ofthe particulate filter regeneration or the desulfation event. Thetemperature for the particulate filter regeneration and the desulfationevent typically is greater than 600° C. Thus, the heat generated in theexhaust system due to the particulate filter regeneration or thedesulfation event may clean the urea deposit off the SCR catalyst. Itshould be noted that urea deposit accumulation may occur quickly in mostof the low operation temperature conditions (e.g., temperature <300°C.). Therefore, the particulate filter regeneration or the desulfationevent may not remove the urea deposit accumulation as desired. The SCRcatalyst regeneration may occur more frequently than the particulatefilter regeneration or the desulfation event. Because the particulatefilter regeneration or the desulfation event is taken into account, theinstance of the regeneration required to remove the urea deposit may bereduced. Thus, energy used for the regeneration of the SCR catalyst maybe decreased, and the duration of normal operation of the SCR catalystmay be increased.

Continuing from either step 226 or step 224, at 228, the method mayinclude estimating urea deposit accumulation during and/or after the SCRregeneration. In some embodiments, the following equation may be used tomake the estimation:

The urea deposit accumulation (during and/or afterregeneration)=estimated or measured urea deposit accumulation before theregeneration—estimated amount of urea deposit removed.

The urea deposit accumulation may be estimated or measured using theapproaches described in steps 208 and 210, respectively. The amount ofurea deposit removed by the regeneration process may be estimated,similarly.

After regeneration is completed, at 230, the method may include coolingdown the SCR catalyst. Next, at 232, the method includes determining aneed to inject an excess amount of urea into the SCR catalyst. Thisdetermination may include determining whether the urea depositaccumulation during and or after the SCR regeneration is below apredetermined residue amount at 234. The predetermined urea depositamount at 234 may be a level at which the SCR catalyst may not performas desired because the amount of stored ammonia is not suitable fordesired NOx reduction. Alternatively, the determination may includedetermining whether the predetermined regeneration time interval isexceeded at 236.

Typically, urea solution needs to be injected continuously, includingduring regeneration, into the SCR catalyst to remove the urea deposits.The exception may be a period during a particulate filter regenerationor desulfation event where the SCR temperature is above 600° C. This isbecause the SCR catalyst may not have activity at temperatures above650° C. In some embodiments, the SCR catalyst may have preferred (orbest) performance for NOx reduction at a temperature (e.g., less than300° C.) when the SCR catalyst stores sufficient ammonia (NH₃) whichcomes from urea decomposition. However, the stored ammonia may bedecreased during regeneration as the SCR catalyst is heated to theregeneration temperature (e.g., in the range of 360 to 450° C.). Thus,an excess amount of urea injection may be desired after SCRregeneration.

Thus, at 238, the method includes injecting the excess amount of ureainto the SCR catalyst. The excess amount of urea injection may bedetermined based on engine operating conditions, such as, speed, load,catalyst temperature, mass airflow, etc.

It should be noted that steps 224 to 238 may be performed at someconditions where the temperature of the SCR catalyst is closed to theregeneration temperature by opportunity. At these conditions, the excessamount of urea may be needed to maintain the desired NOx reduction orconversion efficiency in the SCR catalyst.

The method according to the present disclosure may increase NOxconversion efficiency of the SCR catalyst by removing urea relateddeposits through regeneration. The engine performance and fuel economymay also be improved by reducing the back pressure increase due to clogsby urea deposits. Further, the method takes into account of the removalof urea deposit by the particulate filter regeneration or thedesulfation event, thereby reducing the time required for SCR catalystregeneration based on urea deposit accumulation. Moreover, the NOxbreakthrough during cool down after regeneration (e.g., SCR catalystregeneration, particulate filter regeneration) may be minimized byadjusting the ammonia injection after a regeneration event.

As will be appreciated by one of ordinary skill in the art, the specificroutines and block diagrams described above in the flowcharts mayrepresent one or more of any number of processing strategies such as,event-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Likewise, the order of processing is not necessarily required to achievethe features and advantages of the disclosure, but is provided for easeof illustration and description. Although not explicitly illustrated,one of ordinary skill in the art will recognize that one or more of theillustrated steps or functions may be repeatedly performed depending onthe particular strategy being used. Further, these Figures graphicallyrepresent code to be programmed into the computer readable storagemedium in controller 12.

It will be appreciated that the processes disclosed herein are exemplaryin nature, and that these specific embodiments are not to be consideredin a limiting sense, because numerous variations are possible. Thesubject matter of the present disclosure includes all novel andnon-obvious combinations and subcombinations of the various camshaftand/or valve timings, fuel injection timings, and other features,functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the injection and valve timingand temperature methods, processes, apparatuses, and/or other features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

The invention claimed is:
 1. A method, comprising: directing engine exhaust to a urea-based selective catalyst reduction (SCR) catalyst and then to a particulate filter; regenerating the particulate filter; and adjusting operating parameters to regenerate the SCR catalyst, including varying each of a SCR regeneration duration (longer/shorter) and a SCR regeneration temperature (lower/higher) based on an accumulated urea deposit amount on the SCR catalyst prior to its regeneration.
 2. The method of claim 1 further comprising determining that a threshold for removal of urea deposit accumulation on the SCR catalyst has been met, and wherein upon making such determination, regenerating the SCR catalyst at a predetermined temperature for a predetermined time in response to the determination, and wherein the threshold is based on a pressure drop across the SCR catalyst.
 3. A method, comprising: directing engine exhaust to an oxidation catalyst, then a urea-based selective catalyst reduction (SCR) catalyst, and then to a particulate filter; regenerating the particulate filter; completing the particulate filter regeneration; and then regenerating the SCR catalyst, including varying each of a SCR regeneration duration (longer/shorter) and a SCR regeneration temperature (lower/higher) based on an accumulated urea deposit amount on the SCR catalyst prior to its regeneration.
 4. A method, comprising: directing engine exhaust to an aftertreatment device coupled downstream of the engine, the aftertreatment device including an oxidation catalyst, a urea-based selective catalyst reduction (SCR) catalyst, and a particulate filter; establishing a threshold urea deposit accumulation for regeneration of the SCR catalyst; determining that the threshold urea deposit accumulation in the SCR catalyst has been met; in response to the determination, regenerating the SCR catalyst by maintaining the SCR catalyst at a predetermined regeneration temperature for a predetermined interval, and after determining that the threshold urea deposit accumulation in the SCR catalyst has been met, delaying the SCR regeneration until a regeneration performed in the particulate filter is completed.
 5. The method of claim 4, wherein determining that the threshold urea deposit accumulation in the SCR catalyst has been met includes measuring NOx emissions using a NOx sensor and one of an operating temperature, a urea flow rate, an exhaust flow rate, an exhaust pressure, and a pressure difference across the SCR catalyst.
 6. The method of claim 4, wherein determining that the threshold urea deposit accumulation in the SCR catalyst has been met includes detecting that a pressure drop across the SCR catalyst meets or exceeds a predetermined pressure drop threshold corresponding to the threshold urea deposit accumulation.
 7. The method of claim 4, wherein the predetermined regeneration temperature is 360-450° C.
 8. The method of claim 4, wherein the predetermined interval is at least two minutes.
 9. The method of claim 4, wherein a temperature of the SCR catalyst is raised to and maintained at the regeneration temperature by heating the SCR catalyst using a SCR catalyst heater.
 10. The method of claim 4, wherein a temperature of the SCR catalyst is raised to and maintained at the regeneration temperature by injecting fuel to the oxidation catalyst to generate an exothermic reaction.
 11. The method of claim 4, wherein determining that the threshold urea deposit accumulation in the SCR catalyst has been met includes: estimating urea deposit accumulation based on operation conditions of the SCR catalyst, and determining that the estimated urea deposit accumulation meets or exceeds the threshold urea deposit accumulation.
 12. The method of claim 11, wherein the operating conditions include one of an operating temperature, a urea flow rate, an exhaust flow rate, an exhaust pressure, and a pressure difference across the SCR catalyst. 